Blender is only adding about half of a video?
Either there is something different in 1.mp4 or there something wrong in the Blender import of them. 

Try to compare the metadata of the clips in MediaInfo if they are similar, if they're not, then that might be the reason.

If they are similar, try to make four copies of one of the good clips, and import them in Blender, if the problem is visible, then do a bug report: Menus > Help > Report a bug.